1. Field of the Invention
The present invention relates to a tuned antenna resonant circuit of a passive transponder.
2. Description of the Related Technology
Passive transponders are used in the field of contactless communication for identification (RFID). For this purpose, the carrier wave transmitted by the base station is modulated by the transponder. As passive transponders do not have their own power supply, the energy for the integrated circuit of the passive transponder has to be drawn from the carrier wave by absorption modulation. In the currently used passive 125 kHz systems, this is achieved in the electromagnetic near-field region by an inductive coupling. The energetic range achieved in this way lies in the range from a few cm to around 0.5 m and is dependent upon the prevailing national HF regulations. With the further increasing security requirements placed upon the identification, ever higher data transmission rates are required in order to maintain short identification times. This can only be achieved with high carrier frequencies in the UHF (868 MHz) or microwave (2.45 GHz) ranges. Dipole antennas are usually used for these frequencies. The objective of the development in this field is to achieve long ranges with passive systems and acceptable transmit powers of the base station.
From HF technology it is known, as described for example in RFID Handbook 2nd edition 2000, p.121, that the length of the dipole antenna has to be matched to the wavelength of the sender in order to draw energy from the field of the sender. A complete absorption of the received electromagnetic wave is achieved by matching the impedance to the wave impedance defined by the design of the antenna. If the transponder antenna is located at a fairly long distance from the sender, the antenna voltage resulting therefrom is too low to generate a supply voltage for the transponder by a simple rectification. Because of the HF regulations in the European countries, the ranges of existing UHF transponders lie below one meter.